1. Field of the Invention
The present invention relates to methods, compositions and lightweight additives for sealing pipe strings in well bores, and more particularly, to such methods, compositions and additives wherein the well bores penetrate formations that readily fracture at low hydrostatic pressures.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in oil, gas and water well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casing and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior of a string of pipe disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
In some well locations, the subterranean zones or formations into or through which wells are drilled have high permeabilities and low compressive and tensile strengths. As a result, the resistance of the zones or formations to shear are low and they have low fracture gradients. When a hydraulic cement composition is introduced into a well bore penetrating such a subterranean zone or formation, the hydrostatic pressure exerted on the walls of the well bore can exceed the fracture gradient of the zone or formation and cause fractures to be formed in the zone or formation into which the cement composition is lost. While lightweight cement compositions have been developed and used, subterranean zones or formations are often encountered which have fracture gradients too low for the lightweight cement compositions to be utilized without the formation of fractures and the occurrence of lost circulation problems.
Thus, there are needs for improved lightweight cement compositions for sealing pipe such as casings and liners in well bores which penetrate zones or formations having very low fracture gradients.
The present invention provides lightweight well cement compositions, additives for use in the compositions and methods of using the lightweight compositions for sealing pipe in well bores penetrating zones or formations having low fracture gradients which meet the needs described above and overcome the deficiencies of the prior art. The methods of this invention basically comprise the steps of providing a lightweight cement composition comprised of a hydraulic cement, water and an additive comprising a suspension of microspheres in water gelled or thickened with a water swellable clay suspending agent. The cement composition is placed into the annulus between a pipe and the walls of a well bore and the sealing composition is allowed to set into a hard impermeable mass.
A lightweight sealing composition of this invention is basically comprised of a hydraulic cement, water and an additive for making the cement composition lightweight.
The additives of this invention are comprised of aqueous suspensions of microspheres gelled or thickened with a water swellable clay suspending agent.
It is, therefore, a general object of the present invention to provide methods, lightweight well cement compositions and additives for use in cementing pipe in well bores.